In the event of a total breakdown of an electrical transmission system (AC network), selected power-generating units have the capability for restoring the network. On account of the very limited size of the network, it is very difficult, however, to maintain the balance between power generation and power consumption. As a consequence thereof, frequency changes occur, which are considerable compared with the normal conditions for the network operation.
Independently of the type of plant which is provided and set up for carrying out such a black start, the capability for accepting the immediate satisfaction of demand blocks, which lie typically within the range of 30-50 MW, is required. The power-generating unit must be in the position to control the frequency and voltage level within acceptable limits when satisfying such blocks.
Large power plants are particularly suitable for the restoration of networks. An example of such a suitable power plant is a combined cycle power plant, as is shown schematically and greatly simplified in FIG. 1. The combined cycle power plant 10 of FIG. 1 comprises a gas turbine 11 and a water-steam cycle 12 which are interlinked via a heat restoration steam generator 13. The gas turbine 11 draws in air 19 by a compressor 15, compresses this, and discharges it to a combustion chamber 16, to which a fuel 20 is fed. The fuel 20 is combusted with the aid of the compressed air and produces a hot gas which is expanded in a subsequent turbine 17, performing work. In the process, the turbine 17 drives the compressor 15 on one side and, on the other side, drives a generator 18 which generates alternating current or alternating voltage. The flue gas 21 which issues from the turbine 17 is directed through the heat restoration steam generator 13 and discharges it to the environment via a flue gas stack 22.
Arranged in the heat restoration steam generator 13 is an evaporator 27 of the water-steam cycle 12 in which the water delivered by a feedwater pump 26 is evaporated. The steam which is produced is expanded in a steam turbine 23, performing work, and drives an additional generator 24 for current or voltage generation. The steam which issues from the steam turbine 23 is condensed in a condenser 25 and, completing the cycle, is fed to the feedwater pump 26.
The current (alternating current) which is generated by the generators 18 and 24 is fed into an AC network 28 which is connected to the combined cycle power plant 10. A control unit 14 ensures that the requirements of the AC network 28 are fulfilled as far as possible in the process with regard to frequency and level of output.
A method for the primary control of a combined cycle power plant operating on the network is generally known from printed publication US 2009/0320493 A1, in which by the corresponding operation of a valve which acts upon the steam turbine a reserve power of the steam turbine is constantly held in readiness in order to boost the network frequency in the event of a frequency drop in the network. The case of a black start is not taken into consideration in this case.
In the case of gas-turbine modules of a combined cycle power plant, the transient operation of the gas turbine can lead to significant changes in the exhaust temperature. This generally necessitates the arrangement of a bypass stack. This can be an important point when judging whether a gas-turbine module of a combined cycle power plant is suitable for a black start or not.
Modern high-performance gas turbines are particularly well-suited for network restoration. The flexibility during the operation of these machines within the range of low outputs, however, is restricted in general by process limitations. In the general case, an individual unit cannot therefore fulfill the aforesaid requirements for the entire operating range.